slub: Handle freelist cycle in on_freelist()

slub: Handle freelist cycle in on_freelist()

The on_freelist() doesn't have a way to handle the edgecase of having a
full freelist that doesn't end in NULL and instead has another valid
pointer in the slab as a result of a Use-After-Free or anything similar.

This case won't get caught by check_valid_pointer() and it will result in
nr incrementing to `slab->objects + 1`, corrupting the slab->inuse entry
later in the code by setting it to -1.

Add an if check to detect that case, report it and handle the freelist
and slab appropriately, as is the standard process in these situations.

Furthermore change the return type of the function from int to bool as
per coding style guidelines.

Also move the `break;` line inside the `if (object) {` to make it more
obvious that the code breaks the while loop in that branch.

Signed-off-by: Lilith Persefoni Gkini <lilithgkini@proton.me>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>